The Global Water Cycle

The movement of water through the land, atmosphere, and ocean is termed the global water cycle. The water cycle is intimately tied to the climate of the planet through processes including latent heat exchange and the radiative effects of water vapor. Similarly, climate influences the water resources of the planet through changes in evaporation and precipitation. In examining these processes, long-distance atmospheric transport of water, along with evaporation and precipitation, are the principal inputs in hydrological process and water-resource models. The study of the global water cycle is the unifying theme that can bridge the gap in the spatial-scale spectrum between atmospheric and hydrological sciences. This issue is in its first year and will be implemented through coordinated U.S. and international programs. Planning is underway to develop joint interagency programs in the U.S. and coordination with international programs [e.g., the Global Energy and Water Cycle Experiment (GEWEX), the Program on Climate Variability and Predictability (CLIVAR), Biological Aspects of the Hydrologic Cycle (BAHC), and potentially a more fully coordinated international Hydrology and Water Cycle Program].

    The primary goal of this research is a greater understanding of the seasonal, annual, and interannual mean state and variability of water and energy cycles at continental-to-global scales, and thus a greater understanding of the interactions among the terrestrial, atmospheric, and oceanic hydrosphere in the Earth’s climate system.

    This understanding will be achieved through a combination of observations, modeling, and analysis at a range of spatial and temporal scales, and will provide the foundations for understanding the relationship between weather (the manifestation of fast atmospheric hydrologic processes) and climate (the long-term statistical measures of these hydrological processes.) The research program aims at furthering our understanding of these relationships — especially the relationship between the physical representation of fast hydrologic processes and climatic statistics; the relative roles of land, atmosphere, and ocean hydrologic processes in weather and climate at continental-to-global scales, from daily to interannual timescales; and a determination of how these relationships and roles vary globally and seasonally. Such advances should lead to improved inferences about the occurrence of severe weather events, such as floods and drought, that directly affect property and human safety, and permit the downscaling of hydrological variables (precipitation, surface meteorology, etc.) that can lead to improved water and environmental management.

    An important element of the research program is a quantitative assessment of the improved understanding for weather prediction and for water and environmental management. In addition, advances in understanding the relationships between hydrologic processes and climate will lead directly to better inferences regarding climate change and its subsequent hydrologic impacts at regional-to-global scales. Improving this understanding is hampered by the complexity of the nonlinear hydrologic processes, and in the heterogeneity related to both process forcings and process parameters that exists at all spatial and temporal scales. Understanding is also hampered by a lack of consistent, systematic observations, making it difficult to develop and test new theories and hypotheses regarding the global water cycle.

    Key research challenges include:
  1. Land Surface Interactions: Developing a better understanding of the coupling of land surface hydrologic processes to atmospheric processes over a range of spatial and temporal scales; the role of the land surface in climate variability and climatic extremes; and the role of the land surface in climate change and terrestrial productivity.
  2. Atmospheric Processes: Developing a better understanding of the role of clouds and their influence in the coupling of the atmospheric water and energy cycles, and of the vertical transport and mixing of water vapor on scales ranging from the local boundary layer to regional weather systems.


Tropical Rainfall Measuring Mission

Figure 6. Hurricane Bonnie Storm Cloud, August 22,1998
(See Appendix E for additional information)